From: Thomas Renninger <trenn@suse.de>
Organization: SUSE Products GmbH
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Subject: Re: [PATCH] acpi, cpuidle: Register with cpuidle even if cpu is onlined after boot (beyond maxcpus)
Date: Tue, 26 Jun 2012 11:29:44 +0200
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Cc: Daniel Lezcano <daniel.lezcano@linaro.org>,
        Deepthi Dharwar <deepthi@linux.vnet.ibm.com>,
        linux-acpi@vger.kernel.org, linux-pm@lists.linux-foundation.org,
        Linux PM mailing list <linux-pm@vger.kernel.org>, lenb@kernel.org,
        "Rafael J. Wysocki" <rjw@sisk.pl>, x86@kernel.org,
        linux-kernel@vger.kernel.org
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On Monday, June 25, 2012 06:03:42 PM Srivatsa S. Bhat wrote:
> On 06/25/2012 07:23 PM, Thomas Renninger wrote:
> 
> > On Monday, June 25, 2012 01:25:43 PM Srivatsa S. Bhat wrote:
> >>
> >> Daniel Lezcano noticed that after booting with maxcpus=X, if we online the
> >> remaining cpus by writing: echo 1 > /sys/devices/system/cpu/cpuY/online, then
> >> for the newly onlined cpus, the cpuidle directory is not found under
> >> /sys/devices/system/cpu/cpuY.
> >>
> >> Partly, the reason for this is that acpi restricts the initialization to cpus
> >> within the maxcpus limit. (See commit 75cbfb9 "ACPI: Do not try to set up acpi
> >> processor stuff on cores exceeding maxcpus="). The maxcpus= kernel parameter is
> >> used to restrict the number of cpus brought up during boot. That doesn't mean
> >> that we should hard restrict the bring up of the remaining cpus later on.
> > 
> > Sorry, but IMO it exaclty does mean that (adding more general lists for
> > further comments).
> > 
> > If you can online more cores than maxcpus= via sysfs, this sounds like a bug.
> > Not the other way around.
> > 
> > Compare with Documentation/kernel-parameters.txt:
> >         maxcpus=        [SMP] Maximum number of processors that an SMP kernel
> >                         should make use of.  maxcpus=n : n >= 0 limits the
> >                         kernel to using 'n' processors.  n=0 is a special case,
> >                         it is equivalent to "nosmp", which also disables
> >                         the IO APIC.
> > 
> > Chances that you run into more problems are high.
> 
> 
> Right, I agree on that. So, IMHO, maxcpus=X doesn't mean that the kernel must and
> should forbid any new cpus from coming online, but in the interest of avoiding
> problems/complications in some obscure paths, I guess it makes sense to avoid
> onlining new cpus beyond maxcpus.

Yep, for such reasons:
   - That nobody realizes this to be useful and makes use of it in a productive
     environment
   - If I see maxcpus=X in a bugreport's dmesg command line,
     I want to be sure that's true.
   - To enforce that things work as documented


Wow, after looking a bit into this I found (Documentation/cpu-hotplug.txt):

maxcpus=n    Restrict boot time cpus to n. Say if you have 4 cpus, using
             maxcpus=2 will only boot 2. You can choose to bring the
             other cpus later online, read FAQ's for more info.

Looks like someone already documented this (IMO broken) behavior.
I didn't find further info in the FAQs.

> In any case, I was just trying to see why the simple removal of the setup_max_cpus
> check in acpi_processor_add() wasn't enough to expose the cpuidle directories under
> the new cpus.. and while debugging that, I came up with this patch. I don't mind
> if this doesn't get picked up.

> Right, the usecase of why somebody would like to online new cpus beyond maxcpus
> doesn't look all that solid anyway. So I am OK with leaving the code as it is now.

In the end this is a debug option, I expect everybody is aware of that.
Yep, let's just leave it...

   Thomas
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